The present invention relates to Schottky diodes that can be applied to, e.g., power devices.
Group III nitride semiconductors represented by gallium nitride (GaN) are wide-gap semiconductors having a wide bandgap. For example, GaN and aluminum nitride (AlN) have a bandgap as wide as 3.4 eV and 6.2 eV at room temperature, respectively. Nitride semiconductors are therefore characterized by their higher breakdown field and higher saturated electron drift velocity than those of other compound semiconductors such as gallium arsenide (GaAs), silicon (Si) semiconductors, etc.
In an AlGaN/GaN heterostructure, charges are generated on a (0001)-plane heterointerface by spontaneous polarization and piezoelectric polarization, and a sheet carrier concentration of 1×1013 cm−2 or more is obtained even in an undoped state. A higher current density diode or hetero-junction field effect transistor (HFET) can therefore be implemented by using a two-dimensional electron gas (2DEG) at a heterointerface. Accordingly, power devices using such nitride semiconductors that are advantageous for implementing higher output and higher breakdown voltage are actively being studied and developed.
For example, the general formula “AlGaN” represents a three-element alloy crystal “AlxGa1-xN” (where 0≦x≦1). Similarly, a multi-element alloy crystal is simply represented by a sequence of chemical symbols for constituent elements, such as AlInN or GaInN. For example, a nitride semiconductor “AlxGa1-x-yInyN” (where 0≦x≦1 and 0≦y≦1) is simply represented by AlGaInN.
Schottky diodes are one of the diodes used as power devices. Regarding GaN-based diodes, Schottky diodes using an AlGaN/GaN heterostructure are being developed. GaN-based Schottky diodes are capable of operating with a high current and low resistance because they use as a channel a two-dimensional electron gas layer that is produced at the interface between an undoped AlGaN layer and an undoped GaN layer.
Typically, the Schottky diodes have better switching characteristics but have a greater reverse leakage current as compared to PN diodes. Normally, a passivation film is formed on the device surface as a surface protection film. The passivation film is capable of suppressing surface level formation and reducing a decrease in forward current called current collapse. Since the passivation film further functions to prevent impurities from entering the device, it is essential even in terms of reliability to form a passivation film on the device surface.
Japanese Patent Publication No. 2009-076866 describes a Schottky barrier diode having an increased reverse breakdown voltage.